.back-to-top {
  position: fixed;
  left: 50%;
  margin-left: 580px;
  bottom: 37px;
  border-radius: .2rem;
  border: 1px solid #ccc;
  background-color: rgba(255, 255, 255, .9);
  visibility: hidden;
  opacity: 0;
  transition: 0.4s ease;
  a {
    display: block;
    width: 38px;
    height: 38px;
    line-height: 38px;
    text-align: center;
    font-size: 20px;
    color: #b4b4b4;
  }
}

.back-to-top-show {
  visibility: visible;
  opacity: 1;
  &:hover {
    opacity: 1
  }
}

@media screen and (max-width: 1200px) and (min-width: 770px) {
  .back-to-top {
    left: 50%;
    margin-left: 45.5%;
  }
}

@media screen and (max-width: 770px) {
  .back-to-top {
    right: 2.5%;
    left: auto;
    // left: 50%;
    margin-left: 0px;
    a {
      width: 34px;
      height: 33px;
      line-height: 33px;
      text-align: center;
      font-size: 18px;
    }
  }
}
